Twenty-two people have been killed and 59 injured in a suicide bombing at Manchester Arena at the end of a concert by US singer Ariana Grande.
A lone attacker set off a homemade bomb in the foyer at 22:33 BST on Monday in what Theresa May called an “appalling, sickening” terrorist act.
Armed police have arrested a 23-year-old man in Chorlton, south Manchester, in connection with the attack.
The first victim has been named as 18-year-old student Georgina Callander.
She was studying health and social care at Runshaw College in Leyland, Lancashire.
Relatives are using social media to hunt for loved ones, and an emergency number, 0161 856 9400, has been set up.
In a statement in Downing Street, the prime minister said it was “now beyond doubt that the people of Manchester and of this country have fallen victim to a callous terrorist attack” that targeted “defenceless young people”.
She said the security services believe they know the attacker’s identity but are not yet able to confirm it.
Mrs May has chaired a meeting of the government’s emergency Cobra committee and is expected to travel to Manchester later.
It is the worst terrorist attack in the UK since the 7 July bombings in 2005 in which 52 people were killed.
The Queen has expressed her “deepest sympathy” with all those affected, adding that “the whole nation has been shocked by the death and injury”.
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Sixty ambulances went to Manchester Arena after the attack and those wounded are being treated at eight hospitals around the city.
Greater Manchester Police Chief Constable Ian Hopkins said it was “the most horrific incident” the city had ever faced, and the “fast-moving investigation” was now working to establish whether the attacker “was acting alone or as part of a network”.
The Arndale shopping centre in Manchester was briefly evacuated on Tuesday, but is now reopening.
Eyewitnesses at the Arena described seeing metal nuts and bolts among the debris, and spoke about the fear and confusion that gripped the concertgoers.
Andy Holey, who had gone to the arena to pick up his wife and daughter, said: “An explosion went off and it threw me about 30ft from one set of doors to the other set of doors.
“When I got up I saw bodies lying on the ground. My first thought was to go into the arena to try to find my family.”
Emma Johnson said she and her husband were at the arena to pick up her children, aged 15 and 17.
“We were stood at the top of the stairs and the glass exploded – it was near to where they were selling the merchandise,” she told BBC Radio Manchester.
“The whole building shook. There was a blast and then a flash of fire afterwards. There were bodies everywhere.”
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Teenager Abigail Walker, who was at the concert, told the BBC: “I had to make sure I had my sister. I grabbed hold of her and pulled hard. Everyone was running and crying.
“We were just trying to figure where everyone was. It was absolutely terrifying.”
Charlotte Campbell’s daughter Olivia has been missing since the concert.
“She’s only a 15-year-old girl, she’s out there on her own because her friend has been found,” she told the BBC.
“If anyone sees her contact me. Give her your phone and let her ring me. I just want her home.”
Analysis: Gordon Corera, BBC security correspondent
The UK threat level has been has been judged to be severe for nearly three years – which means an attack is considered highly likely.
But in recent months the tempo of counter terrorist activity has been increasing with – on average – an arrest every day.
After the attack in Westminster by Khalid Masood in March, police and security officials have been warning that further attacks were almost inevitable.
But they also believed that those were more likely to be low-tech involving knives or vehicles. The fact that the Manchester attack involved explosives will worry them.
It may not have been at the level of complexity seen in Paris in 2015, when multiple attackers sent from Syria used guns and suicide belts, but it will still have required planning.
In other developments:
- The explosion happened shortly after Ariana Grande left the stage and afterwards the 23-year-old teen actress-turned-singer, tweeted: “broken. from the bottom of my heart, i am so so sorry. i don’t have words”
- Flags are flying at half mast outside Number 10 and political parties have suspended general election campaigning.
- London Mayor Sadiq Khan said the police presence in the capital would be stepped up
- World leaders have expressed solidarity with the UK, including US President Donald Trump, who called those responsible “evil losers”.
- Exam boards are telling schools they can re-arrange GCSE and A-level exams in the wake of the attack.
- Police have established a help centre at Manchester City’s Etihad Stadium, access Gate 11, for anyone who needs assistance in tracing loved ones.
- Twitter has been flooded with appeals from relatives and friends of missing concertgoers via the hashtag #MissingInManchester, and Facebook has activated a safety check feature so that people can let relatives know they are safe.